Monatin [4-(indole-3-yl-methyl)-4-hydroxy-glutamic acid] is a compound that is one of amino acids contained in roots of Schlerochitom ilicifolius that is a shrub in South Africa and is particularly expected as a low calorie sweetener because of having sweetness one thousand and several hundreds times sweeter than sucrose (see Patent Document 1). The Monatin has asymmetric carbon atoms at positions 2 and 4, and a naturally occurring stereoisomer of Monatin is a 2S,4S-isomer. Naturally non-occurring three stereoisomers have been synthesized by organic chemistry processes. All of these stereoisomers are excellent in sweetness, and expected to be used as the sweeteners.
Several methods have been reported as the methods for producing the Monatin (e.g., see Patent Document 2). However, all of the reported methods require a step of multiple stages, and thus, it is required to improve a synthetic yield of the Monatin.
Specifically, for the method for producing the Monatin, the following method for producing 2R,4R-Monatin by synthesizing indole-3-pyruvate (hereinafter referred to as “IPA” as needed) from L-tryptophan (L-Trp), synthesizing 4R form of 4-(indole-3-yl-methyl)-4-hydroxy-2-oxoglutaric acid (hereinafter referred to as “4R-IHOG” as needed) from the resulting IPA and pyruvate, and subsequently subjecting the obtained 4R-IHOG to an oximation reaction, a reduction reaction and an epimerization-crystallization method has been known (conventional method (1)) (see Patent Document 2).
However, an aldolase step (second step) is an equilibrium reaction, and thus, a satisfactory yield is not always obtained in this reaction.

In order to improve the yield of the 2R,4R-Monatin, the method for producing the 2R,4R-Monatin by a one-pot enzymatic reaction has been invented (conventional method (2)) (see Patent Documents 3 to 6).    Patent Document 1: JP Sho-64-25757-A    Patent Document 2: International Publication WO2003/059865    Patent Document 3: International Publication WO2007/133184    Patent Document 4: International Publication WO2005/042756    Patent Document 5: US Patent Application Publication No. 2006/0252135 Specification    Patent Document 6: US Patent Application Publication No. 2008/020434 Specification